A motorcycle is a two-wheeled vehicle powered by an engine. The wheels are in-line, and at higher speed the motorcycle remains upright and stable by virtue of gyroscopic forces; at lower speeds readjustment of the steering by the rider gives stability. The rider sits astride the vehicle on a seat, with hands on a set of handlebars which are used to steer the motorcycle, in conjunction with the rider shifting his weight through his feet, which are supported on a set of “footpegs” or “pegs” which stick out from the frame. The chassis or frame of a motorcycle is typically made from welded struts, with the rear suspension often being an integral component in the design.
The engine of the motorcycle typically sits under a fuel tank, between the rider's legs. Typically, motorcycle engines displace between about three cubic inches (approximately 50 cubic centimeters) and 140 cubic inches (approximately 2300 cubic centimeters) and have one to four cylinders arranged in a “V” configuration, an in-line configuration or a boxer configuration. In most one-cylinder motorcycle engines, the cylinder points up and slightly forward with a spark plug on top. The most common configuration for two-cylinder motorcycle engines is a “V-twin” where the cylinders form a “V” around the crankshaft, which is oriented transversely (i.e., perpendicular to the direction of travel). Typically, the angle of the V is 90 degrees. Other known configurations for two-cylinder motorcycle engines include a “parallel twin” (i.e., in-line configuration) where the cylinders are parallel, and a “boxer twin” (also called a “flat-twin”) where the cylinders are horizontally opposed, and protrude from either side of the frame. Four-cylinder engines are most commonly configured in-line, although “V” and square configurations are also known. Although less common, motorcycle engines having three, six, eight and ten cylinders are known.
Motorcycle engines are typically cooled either with air or water. Air-cooled motorcycle engines rely on ambient air flowing over the engine to disperse heat. The cylinders on air-cooled motorcycle engines are designed with fins to aid in this process. It is believed that air-cooled motorcycle engines are cheaper, simpler and lighter than water-cooled motorcycle engines, which circulate water between a water jacket surrounding the combustion chamber(s) and a radiator that disperses heat transferred from the engine via the circulating water.
The operation of motorcycle engines may either be two-stroke or four-stroke. It is believed that two-stroke engines are mechanically simpler and may be lighter than equivalent four-stroke engines. But four-stroke engines are believed to operate more cleanly, be more reliable, and deliver power over a much broader range of engine speeds.
Rotation of the engine crankshaft is transferred to a transmission, via a clutch and a primary drive. Most motorcycle transmissions have five or six forward gears; only a few motorcycle transmissions are fitted with a reverse gear. The clutch is typically an arrangement of plates stacked in alternating fashion, one geared on the inside to the engine, and the next geared on the outside to the engine output shaft. Whether wet (rotating in engine oil) or dry, the plates are squeezed together by a spring, causing friction buildup between the plates until they rotate as a single unit, thereby driving the transmission via the primary drive. Releasing the clutch spring allows the engine to freewheel with respect to the engine output shaft. The primary drive couples the engine output shaft to an input shaft of the transmission and typically includes either a toothed belt or a chain.
A secondary or final drive from the transmission to the rear wheel of a motorcycle typically includes a chain, although final drives may alternatively include a toothed belt or an enclosed torque shaft in combination with right-angle drive gearing.
Motorcycle manufacturers often also produce all-terrain vehicles or ATVs. These have two or more back wheels, usually two front wheels, an open driver's seat and a motorcycle-type handlebar. The 4-wheeled versions are also called “quads,” “four-wheelers,”“quad bikes” or “quad cycles.” ATVs are often used off-road for recreation and utility. Recreational ATVs are generally small, light, two-wheel-drive vehicles, whereas utility ATVs are generally bigger four-wheel-drive vehicles with the ability to haul small loads on attached racks or small dump beds. Utility ATVs may also tow small trailers. Utility ATVs with 6 wheels include an extra set of wheels at the back to increase the payload capacity, and can be either four-wheel-drive (back wheels driving only) or six-wheel-drive.
Other types of vehicles that use similar engine technology may include amphibious all terrain vehicles, snowmobiles, personal watercraft and light-sport aircraft. An amphibious all terrain vehicle (AATV) typically has four, six or eight wheels, uses a skid-steer steering system, and the rider sits inside a chasis. Generally designed to float, AATVs can go through swamps as well as traverse dry land. A snowmobile is a land vehicle that is propelled by one or two rubber tracks, with skis for steering. Snowmobiles are designed to be operated on snow and ice, but may also be operated on grass or pavement. A personal watercraft, or PWC, is a recreational watercraft that the rider sits or stands on, rather than inside of, as in a boat. Typically, personal watercraft have an inboard engine driving a pump jet, and are designed for one to four passengers. Light-sport aircraft, which are single or two-seat lightweight, slow-flying airplanes, include “ultralights” that are essentially an engine propelled hang glider-style wing below which is suspended a three wheeled cart for the pilot. An ultralight is controlled by shifting the pilot's body weight with respect to a horizontal bar in roughly the same way as a hang glider pilot flies.